Note the ~12.7% volume increase on melting. Heated storage tanks must be sized for the molten volume, not the bag volume.

Molten MAH is extremely low viscosity - take care at pipe joints; any leak will spread rapidly. All flanged connections in molten MAH systems must use chemical-resistant gaskets (PTFE).

Hydration is moderately exothermic - water ingress into hot molten MAH generates heat + vapour; design heated tanks to prevent water contamination absolutely.

When MAH contacts water, it does not simply dissolve - it immediately and irreversibly reacts to form maleic acid:

The resulting maleic acid solution is highly acidic (pH ~1.5 at 10% concentration). Key implications: (1) moisture in storage causes caking and quality loss; (2) MAH added to wet glassware or reactors generates acid immediately; (3) fire-fighting with water on large MAH fires creates maleic acid runoff - contain and neutralise with alkali; (4) for water treatment applications, MAH is first polymerised before contact with the process water.

The twin carbonyl bands (doublet at ~1,780 and ~1,850 cm⁻¹) are the definitive FTIR signature of a cyclic anhydride; if one band is absent or shifted, the ring may be opened (maleic acid contamination).

The ¹H NMR of MAH is exceptionally simple - a single singlet at δ 7.52 ppm for the two equivalent vinyl protons. Any additional peaks indicate impurities (maleic acid at ~6.3 ppm, fumaric acid at ~6.7 ppm).

Maleic acid (formed by MAH hydrolysis) is a strong acid (pKa₁ 1.94) - solutions are highly acidic; standard acid-resistant materials of construction required.

Q1: What is the melting point of maleic anhydride?

The melting point of maleic anhydride is 52.8 °C (127 °F). This is commercially the most important physical property of MAH because it determines whether the material is handled as a solid or as a liquid in any given environment. At typical ambient temperatures of 20–25 °C, MAH is a solid white crystalline material. In summer warehouses, containers, or heated transport vehicles that reach temperatures above 52.8 °C, MAH can partially or fully melt - causing bag deformation, leakage from damaged bags, and potential caking on re-solidification. For industrial processing (UPR synthesis, esterification, SMA polymerisation), MAH must be melted in heated vessels at 60–80 °C before addition to the reactor. The crystallisation point (≥52.5 °C for standard commercial grade; ≥52.6 °C for premium grade) is the key QC specification for purity: a suppressed crystallisation point below 52 °C indicates contamination with maleic acid (formed by hydrolysis from moisture exposure) or fumaric acid, both of which act as freezing-point depressants and are reject criteria.

Q2: What is the boiling point of maleic anhydride?

The boiling point of maleic anhydride at atmospheric pressure (760 mmHg) is 202 °C. This is considerably higher than the typical processing temperatures used in most MAH applications (60–180 °C for UPR synthesis, 60–120 °C for SMA polymerisation), which means that MAH does not boil off or evaporate significantly during normal industrial use. The high boiling point also means that product loss by evaporation from synthesis reactors is minimal at process temperatures, and that MAH can be used as a component in high-temperature bake-cure systems without loss. For distillation or purification purposes - used to upgrade off-spec or recovered MAH - vacuum distillation is employed, reducing the effective boiling point to approximately 80–100 °C at 1–5 mmHg operating pressure, making the purification energy-efficient and avoiding thermal decomposition above 280 °C. The combination of melting point (52.8 °C) and boiling point (202 °C) defines MAH's processing window: it is a low-melting solid that can be easily liquefied and processed as a mobile liquid over a very wide temperature range (53–200 °C) without significant vapour loss.

Q3: What happens when maleic anhydride contacts water?

When maleic anhydride contacts water, it does not simply dissolve - it reacts rapidly and irreversibly to form maleic acid: C₄H₂O₃ + H₂O → HOOC–CH=CH–COOH (maleic acid). This hydrolysis reaction is exothermic (~59 kJ/mol), fast at ambient temperature, and nearly instantaneous with warm water. The resulting maleic acid solution is strongly acidic (pH ~1.5 at 10% concentration, due to maleic acid's first pKa of 1.94). Practical implications of this behaviour: (1) Storage: MAH bags must be kept sealed and dry; even moderate humidity causes surface hydrolysis, caking, and quality loss; bag leaks in rain or high-humidity environments cause significant maleic acid contamination; (2) Process design: water must be completely excluded from heated MAH tanks; if water enters a hot molten MAH tank, violent exothermic hydrolysis + steam generation can cause a boil-over; (3) Fire response: using water on MAH fires generates acidic maleic acid runoff that must be contained and neutralised; CO₂ or dry powder extinguishers are preferred; (4) Quality control: a suppressed crystallisation point (below 52°C) indicates moisture damage has occurred during storage or transport.

Q4: How do I identify maleic anhydride from maleic acid by FTIR?

Maleic anhydride and maleic acid have distinctly different and definitive FTIR spectra. Maleic anhydride: The diagnostic feature is the cyclic anhydride doublet - two strong C=O stretching bands at approximately 1,850 cm⁻¹ (asymmetric) and 1,780 cm⁻¹ (symmetric). These twin carbonyl bands are the hallmark of any cyclic anhydride; both bands must be present to confirm the anhydride ring is intact. Additionally: C=C stretch at 1,630–1,650 cm⁻¹; C–O–C anhydride stretch at 1,000–1,100 cm⁻¹; vinyl =C–H stretch at 3,100–3,150 cm⁻¹. Maleic acid (hydrolysis product): The twin anhydride bands at 1,780 and 1,850 cm⁻¹ are absent; instead, a broad carboxylic acid O–H stretch appears at 2,500–3,300 cm⁻¹ and a single C=O stretch of the diacid appears at ~1,710 cm⁻¹. If your MAH sample shows a shoulder or peak at 1,710 cm⁻¹ alongside the 1,780/1,850 bands, it indicates partial hydrolysis to maleic acid has occurred. For quantification of maleic acid content in MAH, GC-FID analysis of an acetone solution is more precise than FTIR.

Q5: What is the density of maleic anhydride and how do I calculate tank volumes?

Maleic anhydride has two relevant density values: solid density 1.48 g/cm³ at 20 °C, and molten liquid density 1.31 g/cm³ at 60 °C. For volume calculations: 1 metric tonne (1,000 kg) of solid MAH occupies approximately 676 litres (as compact solid). When melted at 60 °C, the same 1,000 kg occupies approximately 763 litres - an 11–13% volume increase on melting. This means heated storage tanks must be sized for the molten density, not the solid density. Example: a heated MAH storage tank designed for 5 MT holding capacity must accommodate at least 3,815 litres (5,000 kg ÷ 1.31 g/cm³ × 1,000 mL/L). Add a 10–15% safety headspace for thermal expansion during temperature variations, bringing the practical tank volume to ~4,200 litres for a 5 MT MAH storage tank. For volumetric metering of molten MAH into reactors, use the temperature-corrected density (1.31 at 60°C, 1.30 at 70°C, 1.28 at 80°C) for accurate mass conversion.

Q6: What QC tests should I run on received maleic anhydride before using it in UPR synthesis?

For UPR synthesis, a minimum three-test incoming QC protocol is recommended and can be completed in under 35 minutes: (1) Visual inspection: confirm white, free-flowing flakes with no yellow/grey discolouration (yellow = iron contamination; grey = catalyst residue) and no large lumps (lumps indicate moisture damage and caking); (2) Crystallisation point (most important test, 20 minutes): melt approximately 5 g of sample in a glass test tube at 65°C; insert a calibrated thermometer; allow to cool slowly while gently stirring; record the temperature at which the first crystals appear - must be ≥52.5°C for standard grade; a crystallisation point of 52.0°C or below indicates maleic acid contamination from moisture exposure and the batch should be rejected or segregated; (3) APHA colour of melt (10 minutes): keep the sample molten at 70°C and compare against APHA platinum-cobalt colour standards - must be ≤30 APHA for standard grade; ≤10 for premium/gel-coat grade. If visual inspection reveals discolouration, additionally run iron by ICP-OES before releasing. These three tests catch the three most common MAH quality failures: moisture damage (crystallisation point), iron contamination (colour + visual), and substitution with lower-grade material (both tests combined). Provide the batch COA to your synthesis team for the exact purity value used in stoichiometric calculations.